This invention relates to the purifying of crystal material, the doping of the material and the growth of crystals.
Bridgeman, Bridgeman-Stockbarger, Czochralski and variations have been used for crystal growth. Depending on the crystal growth method, the crystal type and the crystal size, one has to overcome sets of problems. This invention relates to the purification of the crystal material and the crystal growth process itself.
Crystal size and the quality of the crystal starting material play important roles in the production of scintillation crystals. The starting material labeled xe2x80x9cscintillation gradexe2x80x9d is of five 9""s purity 99.999%. Often the starting material has poor stoichiometry ratio. Growing crystals in a closed type system that have large diameters and up to over 2000 pounds in weight result in crystals that have poor crystal quality. Crystal purity, dopant distribution, defect density and distribution and built-in stress imposed on the crystal during the crystal growth process and the crucible removal may be at unacceptable levels. With the exception of small crystal portions grown at the beginning of the crystal growth, crystals may have lower purity than the starting material. Dopant concentration varies dramatically. That in turn creates uneven light output and decreases the energy resolution of scintillation crystals. When handling large size crystals during the hot transfer, the crystals release large portions of iodine and thallium iodine vapors. Exposure to ambient temperature creates various defects and defect densities in the hot crystals.
The current practices where large barrel-shaped crystals are grown for all applications, regardless of the fact that most applications use rectangular shapes, makes the yields rather low. Scaling up crystal plate sizes from 0.5-1 inch thick slabs cut perpendicular to the crystal length of a barrel-shaped crystal requires large financial investments. At the same time increasing slab geometry increases the crystal production cost by decreasing the growth rate and lowers the crystal quality and yield.
Existing purification methods include supplying a gaseous medium to a surface of a melt carried in a crucible. Those methods require extended times for purification, up in the range of 96 hours. Those methods also ineffectively cure the melt, as lower portions of the melt are never purified.
During melt purification, impurities react with the gas molecules and exit the melt in a gaseous phase. Some impurities react and precipitate from the melt as a sludge. Other reacted impurities float to the surface.
Needs exist for purification systems that remove impurities faster and more efficiently.
These problems and many more remain in the present practices. Needs exist for new approaches for crystal material purification and the crystal growth processes.
Purifying of crystals by reactant gas contact in current systems results in delays and adds significant times to the crystal growth process.
Reactive gas is released through a crystal source material or melt to react with impurities and carry the impurities away as gaseous products or as precipitates or in light or heavy form. The gaseous products are removed by vacuum and the heavy products fall to the bottom of the melt. Light products rise to the top of the melt. After purifying, dopants are added to the melt. The melt moves away from the heater and the crystals formed. Subsequent heating zones re-melt and refine the crystal, and a dopant is added in a final heating zone. The crystal is divided, and divided portions of the crystal are re-heated under pressure for heat treating and annealing.
The invention provides multi zone plate crystal growth and purifying.
The new continuous feed multi-zone crystal grower is capable of growing crystals with very large dimensions under reactive atmospheres. The invention produces high purity crystals with very uniform doping concentrations regardless of the crystal size. The dopant level and the residual impurities are controlled in situ within the crystal feed chamber and during the crystal growth process. Crystal applications include nuclear medicine, high energy physics, optics and others where economical production of high purity and large size crystals are required.
The invention provides horizontal (or inclined under some angle) continuous crystal growth process for plates of any dimensions.
Reactive gas permeates start-up material, crystal powder or polycrystalline material or a crystal melt.
Stoichiometry control or xe2x80x9crepairxe2x80x9d of start-up material is achieved using the present invention.
Multi-zone traveling, stationary immersed and non-immersed heaters, resistive and RF heating elements, or other type heaters are used. This allows controlled gradient crystal growth of any size crystals.
A traveling crucible or crystal slab can be used if the heaters are stationary.
The present invention can be attached as a module to heaters for in situ purification and dopant control.
Dopant concentration control can be achieved by adding dopant in solid or gaseous form. If excess dopant has to be controlled, the excess is either neutralized via chemical reaction or by dilution with pure melt.
For very high purity crystals or crystals with very large sizes, residual impurities control can be achieved by removing the melt from one of the molten zones via vacuum suction and melt draining.
High temperature and high pressure annealing of the plates in final sizes enhances the crystal quality properties.
The invention eliminates cutting of at least one dimension of the crystal before further processing.
A preferred continuous crystal plate growth apparatus has a source of starter material. A valve supplies material from the starter material source. A first, hot zone communicates with the valve for heating the material. A dopant source and a dopant controller are connected to the hot zone for supplying dopant into the material in the hot zone. A second reduced heat zone beyond the hot zone reduces heat in the material, which forms a solid plate. A receiver receives the solid plate from the second, reduced heat zone and advances the solid plate. A lowered temperature heating zone adjacent the receiver lowers temperature of the solid crystal plate on the receiver. An enclosure encloses the zones and the solid crystal plate in a controlled gaseous environment.
A large heater overlies the small heater. The large heater has first and second zones, and the small heater has the first hot and second reduced heat zones. Baffles separate the first and second zones of the heaters.
The first zone of the small heater produces a crystal melt temperature higher than a crystal melting temperature in the material. The second zone of the small heater produces a temperature lower than the melting temperature. The temperature in the material at the small heater baffle is about the melting temperature. The charge heater first zone provides heat below the melting temperature, and the large heater second zone provides a lower heat.
Preferably the receiver is a conveyor which moves at a speed equal to a crystal growth rate.
A second source of starter material and a second valve are connected to the hot zone for flowing material from the second source to the hot zone.
The crystal melt or starter material is purified in a chamber having a bottom and sides. A lid covers the chamber. An opening introduces liquid or solid material into the chamber. An outlet near the bottom of the chamber releases crystal melt or starter material from the chamber. A shut-off valve opens and closes the outlet. A source of reactive gas is connected to the chamber and extends into a bottom of the chamber. A reactive gas release barrier near the bottom of the chamber slowly releases reactive gas into the crystal starter material. A gas space is located at the top of the chamber above the crystal melt or starter material. An exhaust line is connected to the space at the top of the chamber for withdrawing gas from the top of the chamber. A heater adjacent the chamber heats the chamber and the crystal melt or starter material within the chamber.
The heater has heating elements around sides of the chamber and along the walls of the chamber.
The shut-off valve is a thermally activated or a mechanical or electromechanical valve.
An inlet conduit is connected to the lid. A source of reactive liquid or solid is connected to the inlet conduit. A valve is connected between the source of reactive liquid or solid. A plug is connected to the conduit for plugging the conduit after adding reactive liquid or solid to the chamber.
Preferably a vacuum pump is connected to the exhaust line. A preferred barrier is a porous plate.
In one heating and purifying embodiment, a chamber has an inlet and an outlet. A purified material discharge is connected to the outlet. An enclosure has side walls, a bottom and a top. A reactive gas source is connected to a gas inlet tube. A gas distributor is mounted in the chamber near the bottom. A gas releasing plate connected to the gas distributor releases the reactive gas from the inlet tube and the distributor into the material in the feeding and purifying chamber. A heater heats material in the chamber. A gas exhaust exhausts gas from an upper portion of the chamber.
A preferred casing has a cover and side walls, and the casing side walls include the chamber side walls.
In one embodiment, an upper heater has heating elements across a top of the chamber.
The apparatus moves with respect to a stationary base for supporting a growing crystal.
Preferred crystal growth embodiments have a support for supporting a growing crystal. A first zone heater adjacent the growing crystal heat and liquefies the growing crystal. A second zone heater spaced from the first zone heater along the growing crystal re-liquefies the growing crystal. Preferably multiple zone heaters are spaced from each other along the growing crystal for sequentially liquefying the growing crystal. Preferably the first zone heater further includes heating and purifying apparatus for purifying the crystal melt. A preferred first zone heater includes a reactive gas distributor for distributing reactive gas from near a bottom of the crystal melt.
A liquid or solid adaptive substance source releases liquid or solid reactive substance into the melt.
A source of dopant is connected to the last zone heater for supplying dopant into the crystal melt.
In one embodiment the support is a movable support for moving the liquid crystal along zone heaters. Alternatively, the zone heaters move along the crystal.
One crystal growth embodiment has a chamber for holding a crystal melt. A crystal support holds a crystal movable with respect to the chamber for forming a bottom of the chamber with the crystal. A first heater adjacent the chamber heats and maintains a crystal melt within the chamber. A baffle is connected to the first heater adjacent a bottom of the chamber. A second heater is connected to the baffle beyond the first heater. A source of reactive gas feeds a gas tube connected to a controller. A distributor is connected to the gas tube and is mounted in the chamber for positioning within the crystal melt. A gas releaser connected to the distributor releases reactive gas into the crystal melt. A gas exhaust is connected to the chamber exhausts gas from the chamber above the crystal melt. An inlet tube and a controller release reactant substance into the chamber and into the crystal melt. A dopant conduit and a dopant source provide a dopant from the source through the conduit to the chamber. The reactive substance and the reactive gas control the dopant.
These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings.